Sometimes I find it very difficult to understand why one style of an imitative fly such as a damsel nymph succeeds when a similar one will fail even when both seem to be fair copies of the real insect. The failing one regularly seems to be on my leader while the successful one is tied to the angler next to me!
The Dutch biologist Niko Tinbergen was awarded the Nobel Prize for science for his contributions to the study of animal behaviour (ethology). One of his experimental subjects was a sexually mature stickleback which, as every schoolboy knows, develops a red belly at this time. This fish was observed to dash across his aquarium when a red mail van drove past the laboratory window. The fish was aggressively trying to chase away the reflection of the mail van in the glass side of the aquarium. Tinbergen tried to induce this reaction by making models of male sticklebacks and introducing them to the resident fish. Perfect models of sticklebacks were ignored unless they had a red belly, whereas unrecognisable shapes were attacked if they had a red belly.
When tying trout flies we do need to understand what the trigger is. The small flash of yellow on the thorax of a damsel nymph may be the only difference between success and failure.
The trouble is that once you have discovered the trigger then next time you use it the trout are responding to a different one!! Perhaps somebody will find a way of teaching trout to read the scientific papers.